The authors thank Dr. Terrence R. Burke, Jr. (NCI, NIH) for reading the manuscript and valuable discussions. We also thank Ms. Akane Omagari, Ms. Kiyomi Hashimoto, and Mr. Akira Matsumura for technical assistance in the preparation of cyclic peptides. This work was supported by Health and Labour Sciences Research Grants (Research on HIV/AIDS) and a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Computation time was provided by the Supercomputer Laboratory, Institute for Chemical Research, Kyoto University. S.O. is grateful for a Research Fellowship from the Japan Society for the Promotion of Science for Young Scientists.